The present invention relates to a discharge lamp lighting circuit which controls lighting plural discharge lamps in that ripple current and noise are depressed.
A lighting circuit of a discharge lamp (metal halide lamp etc.) is known, which provides a direct current power source, a direct currentxe2x80x94direct current converting circuit (DC-DC converter), a direct currentxe2x80x94alternating current converting circuit (bridge type circuit, DC-AC converter etc.), a starting circuit, and so on. The following two modes are used for constituting a lighting circuit controlling lighting plural, two for example, discharge lamps. One is a mode in which each discharge lamp is lighted by each circuit preparing two sets of circuit having a DC-DC converting circuit and a DC-AC converting circuit. The other is a mode in which two discharge lamps are lighted using a DC-DC converting circuit obtaining positive polarity output, a DC-DC converting circuit obtaining negative polarity output, and one DC-AC converting circuit.
Since two discharge lamps are controlled to light respectively using one control circuit in any of the modes, it is advantage to simplify constitution and to make low cost. In the case adopting PWM (Pulse Width Modulation) control, control signal is obtained by generating saw tooth wave of a determined frequency, detecting voltage and current applied to the discharge lamps, and comparing arithmetic signal (direct current voltage) calculated based on the detected signal with the saw tooth wave. By carrying out ON/OFF control supplying the control signal to a switching element of each DC-DC converting circuit, duty ratio (or duty cycle) can be provided. (As the result, output voltage of the DC-DC converting circuit is determined.)
Only the main part of such the circuit is shown in FIG. 8, an arithmetic signal based on voltage detecting signal and current detecting signal detected from one discharge lamp (described xe2x80x9cEA1xe2x80x9d) is supplied to a comparator C1 as positive input, and an arithmetic signal based on voltage detecting signal and current detecting signal detected from the other discharge lamp (described xe2x80x9cEA2xe2x80x9d) is supplied to a comparator C2 as positive input.
Although saw tooth wave (described xe2x80x9cSAWxe2x80x9d) supplied to a negative terminal of each of comparators C1 and C2 is generated at an oscillating circuit d using time constant by a resistor RT and a capacitor CT for example, numbers of circuit elements can be decreased by sharing the oscillating circuit in the control circuit. That is, saw tooth wave using for obtaining control signal to one DC-DC converting circuit and saw tooth wave using for obtaining control signal to the other DC-DC converting circuit are made the same signal. Then, by that the result comparing arithmetic signals EA1 and EA2 of every discharge lamp with each of comparators C1 and C2 against the signal (output signals SO1 and SO2) is supplied to individual switching element constituting each DC-DC converting circuit, ON/OFF control thereof is carried out so that each duty ratio is determined.
Since synchronous relation appears at ON state of the switching element about the individual switching element constituting each DC-DC converting circuit in the above-mentioned circuit, the following problems appear:
Increase of ripple current; and
Increase of radio (frequency) noise.
An example of waveform and phase relation about above-mentioned potentials of arithmetic signals EA1 and EA2, waveform of saw tooth wave SAW, output signal SO1 of the comparator C1, and output signal SO2 of the comparator C2 is shown in FIG. 9. Signal SO1 (or SO2) becomes L (low) level when potential of SAW is higher than potential of signal EA1 (or EA2)
Because the switching element of each DC-DC converting circuit is ON/OFF-controlled based on signals SO1 and SO2, ripple current of the capacitor provided at an input stage of the DC-DC converting circuit becomes large so that it causes large capacity and large size of the capacitor and increase of cost when both of element turns ON state or OFF state synchronizing.
Being large in ripple current means the noise component too becomes large responding to it, therefore, addition of elements and parts for depressing noise is unavoidable so as to cause large-size circuit and increase of cost.
Then, an object of the invention is to depress ripple current and noise, to make small size and low cost in a lighting circuit carrying out controlling lighting plural discharge lamps.
In order to solve the above-mentioned problems, the discharge lamp lighting circuit of the invention provides plural DC-DC converting circuits for converting direct current input voltage to designated direct current voltage and controlling lighting plural discharge lamps, wherein state of conductivity or non-conductivity of each switching element constituting the above-mentioned DC-DC converting circuit is controlled as non-synchronization.
According to the invention, ripple current and radio noise can be depressed by controlling so that signals supplied to each switching element constituting the DC-DC converting circuit do not synchronize.